There has been a demand for capacity enlargement of an optical communication system along with an explosive increase in data communication traffic in recent years, and developments in integration and complication of optical components used therein as well as an increase in speed of signals are in progress. Examples of such optical components include optical modulators. A polarization multiplexing optical I/Q modulator in which two optical I/Q modulators (see Non Patent Literature 1, for example), each based on a Mach-Zehnder (MZ) modulator adaptable to multilevel modulation such as QPSK (quadrature phase shift keying) and 16QAM (16 quadrature amplitude modulation), are integrated for two polarized optical waves (a configuration including four Mach-Zehnder modulators integrated in total) has been increasingly used nowadays in order to enlarge a transmission capacity.
This polarization multiplexing optical I/Q modulator can generate an optical modulation signal in the order of 100 Gbits/s. However, in this process, it is necessary to input a high-speed electric signal at a symbol rate of several tens of gigahertz to each Mach-Zehnder modulator in the chip. Usually, a high-frequency signal inputted through an RF interface of a polarization multiplexing optical I/Q modulator module package is passed through a high-frequency wiring board in the module package and eventually supplied to the polarization multiplexing optical I/Q modulator chip. In order to suppress power loss and crosstalk in the above process, it is essential to minimize wiring for connection between the high-frequency wiring board and the polarization multiplexing optical I/Q modulator chip.
To this end, the RF interface of the polarization multiplexing optical I/Q modulator chip needs to be located at an end of the chip by appropriately laying out a high-frequency wiring in the chip. FIG. 1 shows a configuration of a polarization multiplexing optical I/Q modulator in which 50-ohm microstrip lines formed on a SI-InP substrate intersect with InP-based optical waveguides. As a consequence, each high-frequency line 103 connected to a modulation electrode 102 inevitably intersects with the optical waveguides 101 that propagate optical signals.